The present invention relates to a multiple conversion tuner. Such a tuner may be used, for example, in receivers for receiving broadcast signals by cable distribution networks or from satellite or terrestrial aerials.
Multiple conversion tuners are well known for use in receiving radio signals. Such tuners comprise a plurality of cascade-connected frequency changers, each of which converts the frequency of an input signal to an intermediate frequency. For example, in a typical double conversion tuner, the first frequency changer is an up-converter and converts a frequency band containing a plurality of channels from which it is desired to select an input signal to a higher intermediate frequency. The frequency changer has a local oscillator which is tuned so as to convert a desired channel to the nominal first intermediate frequency and a bandpass filter following the first frequency changer selects the desired channel. The output of the filter is supplied to a second frequency changer of the down-converter type. The second frequency changer has a fixed frequency local oscillator and converts the selected channel to a much lower intermediate frequency. The output of the second frequency changer is filtered by another bandpass filter at the second intermediate frequency and supplied to a demodulator.
In order to prevent interaction between the local oscillators of the frequency changers, it is known for each local oscillator to be housed in a Faraday cage so as to suppress radiation from the local oscillator. Such measures are necessary in order to prevent the radiated local oscillator signals from interfering with other signals within the tuner. However, this physical arrangement prevents a complete multiple conversion tuner from being formed in a single monolithic integrated circuit because such integrated circuits cannot provide the electromagnetic screening which is considered necessary to prevent interference resulting from leakage from the local oscillator signal into other parts of the tuner circuit.
Various attempts have been made to integrate a complete tuner circuit in a minimum number of monolithic integrated circuits with minimum screening, despite the screening problems mentioned hereinbefore. It has been found that such arrangements give rise to a new form of interference which has not previously been described. The basic mechanism comprises beating between two or more local oscillator signals producing a low level signal or xe2x80x9cspurxe2x80x9d in the output of one or more of the local oscillators. This spur is supplied, together with the desired local oscillator signal, to the corresponding frequency changer. If there happens to be an input signal to the frequency changer at an appropriate frequency, this is converted so as potentially to lie in the desired channel signal after the frequency conversion. The interfering signal is then passed together with the desired signal through the downstream parts of the tuner circuit and appears in the output of the demodulator as a source of interference.
According to the invention, there is provided a multiple conversion tuner, comprising N cascade-connected frequency changers, where N is an integer greater than one and each ith frequency changer comprises a mixer and a local oscillator, and a local oscillator frequency selecting circuit for selecting the frequencies of at least two of the local oscillators so that:
(i) a desired signal is converted to the Nth intermediate frequency at the output of the Nth frequency changer;
(ii) the frequency band occupied by the desired signal at the output of each ith frequency changer is within the passband of an ith intermediate frequency part of the tuner; and
(iii) there is no signal at a frequency Fi in the input spectrum of the ith mixer such that             IF      i        -                  Bw        i            2        ≤      "LeftBracketingBar"                  F        i            ±              F        s              "RightBracketingBar"    ≤            IF      i        +                  Bw        i            2      
where IFi is the ith intermediate frequency to which the desired signal is converted, Bwi is the bandwidth of the desired signal at the output of the ith frequency changer and Fs is the frequency of any component signal which is generatable by non-linear mixing of the N local oscillator signals.
FS may be expressed as |A1. Flo1xc2x1. . . AN. FloN|, where Floi is the frequency of the ith local oscillator, Ai is an integer greater than or equal to zero and at least two of these integers are non-zero.
The first frequency changer may be an up-converter.
The first frequency changer may be tuneable for selecting the desired signal. The local oscillator of the or at least one frequency changer subsequent to the first frequency changer may have an output frequency which is shiftable by at least one discrete step.
The tuner may comprise a bandpass filter between the first and second frequency changers.
The selecting circuit may comprise a respective look-up table for each of the at least two local oscillators for converting a channel request signal to a local oscillator frequency controlling signal in accordance with a predetermined function.
The selecting circuit may comprise an interference detector and a tuning control arrangement for varying the frequencies of the at least two local oscillators so as to reduce interference detected by the detector. The tuning control arrangement may be arranged to vary the frequencies so as to minimise the interference. The interference detector may comprise a bit error rate estimator. The interference detector may comprise a circuit for measuring television line frequency energy.
It is thus possible to provide a tuner which can be made without substantial special arrangements for electromagnetically screening the local oscillators from each other. For example, such a tuner may be formed in a single monolithically integrated circuit.
The interference mechanism described hereinbefore can be substantially avoided or the effect thereof reduced to such a level as to permit acceptable performance. In particular, it is possible to select the local oscillator frequencies so that the effect of any spur can be substantially avoided by moving potential spurious mixing products out of band.